1. Field of the Invention
The invention relates to a plasma display panel, and more particularly to a plasma display panel capable of enhancing a discharge efficiency.
2. Description of the Related Art
A plasma display panel is structurally grouped into a DC type plasma display panel in which electrodes are exposed to a discharge gas space and which operates in DC discharge condition, and an AC type plasma display panel in which electrodes are covered with a dielectric layer and which operates indirect AC discharge condition. An AC type plasma display panel is predominantly used because it has a longer lifetime and can display images with higher definition.
An AC type plasma display panel is further grouped into a memory operation type panel making use of a memory function of a display cell, and a refresh operation type panel not making use of such a memory function.
A luminance in a plasma display panel is in proportion to the number of discharges.
Since a luminance is reduced, as a display capacity is increased, in a refresh operation type panel, a refresh operation type panel is used as a plasma display panel having a small display capacity.
FIG. 1 is an exploded perspective view of a display cell in a conventional AC type plasma display panel. FIG. 2A is a partial plan view of scanning and sustaining electrodes in the AC type plasma display panel illustrated in FIG. 1, and FIG. 2B is a cross-sectional view taken along the line 2B—2B in FIG. 2A.
In a display cell, there are arranged a first electrically insulating substrate 101 (hereinafter, referred to simply as “first substrate”) and a second electrically insulating substrate 102 (hereinafter, referred to simply as “second substrate”) both composed of glass. The first substrate 101 will make a rear panel substrate, and the second substrate 102 will make a front panel substrate.
The second substrate 102 is formed on a surface facing the first substrate 101 with transparent electrodes 103 and 104 extending in a first direction (a row direction) in parallel with each other.
Bus electrodes 105 and 106 are formed on the transparent electrodes 103 and 104, respectively. For instance, the bus electrodes 105 and 106 are comprised of a thin CrCu film or a thin Cr film having a thickness of about 1 to 4 μm. The bus electrodes 105 and 106 reduce a resistance between the transparent electrodes 103 and 104 and external driver circuits.
The transparent electrode 103 and the bus electrode 105 define a scanning electrode 115, and the transparent electrode 104 and the bus electrode 106 define a sustaining electrode 116.
In each of display cells, the bus electrodes 105 and 106 are located remotest from a surface-discharge gap defined between the transparent electrodes 103 and 104.
The transparent electrodes 103 and 104 are covered with a dielectric layer 112, and the dielectric layer 112 is covered with a protection layer 114. The protection layer 114 is composed of magnesium oxide (MgO), for instance, and protects the dielectric layer 112 from discharges.
The first substrate 101 is formed on a surface facing the second substrate 102 with a data electrode 107 extending in a second direction (a column direction) perpendicular to the first direction.
A dielectric layer 113 is formed on the first substrate 101, covering the data electrode 107 therewith. On the dielectric layer 113 is formed a plurality of partition walls 109 extending in the second direction.
A phosphor layer 111 is formed on sidewalls of the partition walls 109 and an exposed surface of the dielectric layer 113. The phosphor layer 111 converts ultra-violet rays generated by discharges, into visible lights 110.
Between the first and second substrate 101 and 102 and further between adjacent partition walls 109 are defined discharge gas spaces 108 filled with discharge gas comprised of helium, neon or xenon alone or in combination.
In the plasma display panel having the above-mentioned structure, when a voltage difference between the scanning electrode 115 and the sustaining electrode 116 is over a threshold voltage, discharge is generated, and accordingly, the visible light 110 is emitted.
FIG. 3 is a timing chart of an operation of the above-mentioned conventional plasma display panel. Hereinbelow is explained an operation of the conventional plasma display panel with reference to FIG. 3.
As illustrated in FIG. 3, a sub-field is comprised of a priming period, an address period, a sustaining period, and a charge-eliminating period arranged in this order.
In the priming period, a serrate priming pulse Ppr-s is applied to the scanning electrode 115, and a rectangular priming pulse Ppr-c is applied to the sustaining electrode 116. The priming pulse Ppr-s is a positive pulse, whereas the priming pulse Ppr-c is a negative pulse.
According to Electronics-Data Communication Academy Technical Report EID98-95, January 1991, pp. 91, a black luminance can be reduced by using a serrate pulse having a voltage gradient of 7.5 V/microsecond or smaller.
The smaller the gradient is, the more significantly a black luminance can be reduced. However, if a voltage gradient is too small, it would take much time for a voltage to reach a threshold voltage at which a priming discharge is generated. As a result, it is unavoidable that the priming period is long, and hence, the sustaining period is short, causing a problem that a peak luminance in sustaining discharge is lowered, and resultingly, a contrast is lowered. Hence, a voltage gradient of about 4 V/microsecond is usually selected.
In the priming period, a voltage of the scanning electrode 115 is varied by means of a ramp pulse having a voltage gradient of 1 to 10 V/microsecond such that the voltage of the scanning electrode 115 becomes positive relative to voltages of the sustaining electrode 116 and the data electrode 107. After the voltage of the scanning electrode 115 has stopped rising, the voltage is then lowered by means of a ramp pulse having a voltage gradient of 1 to 10 V/microsecond. While the voltage is being lowered, a voltage of the sustaining electrode 116 is raised such that the voltage of the sustaining electrode 116 becomes positive relative to the voltage of the scanning electrode 115. The address period follows the priming period, in which whether images are displayed or not is determined for each of display cells, and the sustaining period follows the address period, in which a luminance at which images are displayed is determined.
The application of the priming pulses Ppr-s and Ppr-c causes generation of priming discharge in the discharge gas space 108 in the vicinity of a discharge gap defined between the scanning and sustaining electrodes 115 and 116, and generation of active ions which facilitate generation of subsequent sustaining discharges. Furthermore, negative wall charges are accumulated on the scanning electrode 115, and positive wall charges are accumulated on the data and sustaining electrodes 107 and 116.
Then, a charge-controlling pulse Ppe-s is applied to the scanning electrode 115. As a result, weak discharge is generated, and resultingly, the negative wall charges accumulated on the scanning electrode 115 and positive wall charges accumulated on the data and sustaining electrodes 107 and 116 are reduced. In particular, wall charges existing in the vicinity of a discharge gap defined between the scanning and sustaining electrodes 115 and 116 are eliminated by the charge-controlling pulse Ppe-s.
In the subsequent address period, a discharge cell or discharge cells from a visible light is to be emitted is(are) selected. Writing discharge is generated only in display cells having been selected by a negative scanning pulse Pbw-s or Pw-s applied to the scanning electrode 115 and a positive data pulse Pd applied to the data electrode 107, resulting in that wall charges are accumulated on electrodes in display cells from which a visible light is to be emitted in the following sustaining period.
In a discharge cell or discharge cell in which the writing discharge has been generated, wall charges are accumulated on electrodes in the discharge cell(s). In contrast, in a discharge cell or discharge cells in which the writing discharge has not been generated, the discharge cell(s) remains in a condition identical to a condition found when the prime period has been terminated.
In the following sustaining period, a visible light is emitted from the selected display cell(s).
A negative sustaining pulse Psus-c is first applied to the sustaining electrode 116, and then, a negative sustaining pulse Psus-s is applied to the scanning electrode 115. The sustaining pulses Psus-c and Psus-s are alternately applied to the sustaining and scanning electrodes 116 and 115. Since wall charges having existed in the vicinity of a discharge gap defined between the scanning and sustaining electrodes 115 and 116 are eliminated in display cells in which the writing discharge was not generated, sustaining discharge is not generated in the display cells, even if the sustaining pulses Psus-s and Psus-c were applied to the display cells.
Since positive wall charges are accumulated on the scanning electrode 115 and negative wall charges are accumulated on the sustaining electrode 116 in a display cell or display cells in which the writing discharge was generated in the address period, a voltage of the negative sustaining pulse Psus-c applied to the sustaining electrode 116 and a voltage caused by the wall charges are added to each other. When a voltage across the scanning and sustaining electrode 115 and 116 is over a threshold voltage at which discharge is generated, there is generated strong discharge.
Once discharge is generated, the wall charges are rearranged so as to cancel voltages applied to the electrodes. Accordingly, negative charges are accumulated on the sustaining electrode 116, and positive charges are accumulated on the scanning electrode 115. A positive-voltage pulse is applied to the scanning electrode 115 as the next sustaining pulse. A voltage of the applied pulse and a voltage caused by the wall charges are added to each other, and when an effective voltage applied to the discharge gas space 108 is over the above-mentioned threshold voltage, discharge is generated.
Hereinafter, the same steps as mentioned above are repeatedly carried out, thereby discharges are repeatedly generated accordingly. A luminance is dependent on the number of repetition of the discharges.
In the subsequent charge-eliminating period, a negative serrate pulse Pse-s is applied to the scanning electrode 115. The application of the pulse Pse-s eliminates wall charges accumulated on the electrodes due to light-emission in the previous sub-field, and uniformizes conditions of all of display cells regardless of whether light-emission was accomplished in the previous sub-field.
Japanese Patent Application Publication No. 11-67100 has suggested a plasma display panel including a partition wall arranged in a matrix, and a bus electrode constituting a scanning electrode which bus electrode is located adjacent to a discharge gap for the purpose of reduction in power consumption. FIG. 5 of the Publication shows that a voltage at which discharge is generated between a scanning electrode and a data electrode is lowest when a bus electrode is located at the middle between a discharge gap and a non-discharge gap of a transparent electrode constituting a scanning electrode. Furthermore, FIG. 6 of the Publication shows that a luminance is reduced to a greater degree because a light is shielded by a bus electrode, if the bus electrode is located closer to the discharge gap from the non-discharge gap. Hence, a bus electrode is positioned so as to have a low-voltage effect which would cancel a disadvantage of the reduction in a luminance. A bus electrode constituting a sustaining electrode is located in the vicinity of the non-discharge gap.
FIG. 4 is a plan view of a display cell in a plasma display panel suggested in Japanese Patent Application Publication No. 2001-236889.
As illustrated in FIG. 4, a surface electrode. 125 is comprised of a plurality of thin-line electrodes 121 extending in a row direction and equally spaced away from one another between a discharge gap 122 and a non-discharge gap 123, and thin-line electrodes 124 extending in a column direction and connecting ends of the thin-line electrodes 121 to one another. Thin-line electrodes 124 extending from the surface electrodes 125 in the column direction and bus electrodes 126 extending in a row direction are electrically connected to each other, and make sustaining electrode pairs including a scanning electrode 127 and a common electrode 128.
In the surface electrode 125 illustrated in FIG. 4, the thin-line electrodes 121 at which sustaining discharge is generated and from which a line of electric force for expanding plasma extends are equally spaced away from one another between the discharge gap 122 and the non-discharge gap 123. The surface electrode 125 having such a structure reduces an intensity of electric field in a discharge space while discharge is being generated, in comparison with the electrode having a non-opening structure, illustrated in FIG. 2A.
Xenon (Xe) gas irradiates ultra-violet rays in a de-excitation step. It is known that if a mixture ratio of Xe gas to mother gas is in the range of about 20 to about 30%, an efficiency of excitation of Xe gas is reduced as an intensity of electric field is increased, even if Xe atoms are attempted to be excited by raising an intensity of electric field when discharge is to be generated. In light of such a phenomenon as mentioned above, it is understood that it is effective to reduce an intensity of electric field in a discharge space during discharge is being generated, in order to enhance an efficiency at which ultra-violet rays are generated, and hence, a light-emission efficiency.
Thus, it would be possible to enhance a light-emission efficiency by generating sustaining discharges by means of the surface electrode 125, and resultingly, reduce power consumption. In addition, since the surface electrode 125 does not extend to adjacent display cells, it would be possible to prevent discharges from being wrongly generated and from being wrongly stopped in being generated, both caused by discharge interference among adjacent display cells.
A recent plasma display panel driven in accordance with pulses having a depressed priming waveform illustrated in FIG. 3 is accompanied with a problem that the scanning and sustaining electrodes 115 and 116 illustrated in FIGS. 2A and 2B are likely to cause excessively eliminate wall charges between the scanning and sustaining electrodes 115 and 116 at the latter half of the priming period, and resultingly, subsequent address discharges are difficult in being generated. This problem is caused by that the scanning and sustaining electrodes 115 and 116 continuously extend from a discharge gap which is defined between the scanning and sustaining electrodes 115 and 116 and at which discharge is generated, to a non-discharge gap.
A recent plasma display panel is accompanied further with a problem that preliminary discharge expands fully in the discharge gas space 108, and resultingly, a luminance in displaying solid black raises.
As suggested in the above-mentioned Japanese Patent Application Publication No. 11-67100, by arranging a bus electrode constituting a scanning electrode, at a center between a discharge gap and a non-discharge gap, wall charges are much accumulated on a dielectric layer above the bus electrode and in the vicinity of the discharge gap.
However, since a charge-eliminating discharge generated by a pulse having a depressed priming waveform expands entirely over the electrodes, the wall charges accumulated on the dielectric layer above the bus electrode and in the vicinity of the discharge gap are likely to be eliminated. Elimination of the wall charges causes that address discharge is unlikely to be generated above the bus electrode and in the vicinity of the discharge gap, and hence, address discharge is generated on the transparent electrodes other than the bus electrode. As a result, it would be impossible to smoothly transfer to sustaining discharge from address discharge.
In the plasma display panel suggested in the above-mentioned Japanese Patent Application Publication No. 2001-236889, illustrated in FIG. 4, the above-mentioned problem is not caused, because both of the scanning and sustaining electrodes extend from a discharge gap towards the bus electrodes 126 in a non-continuous pattern.
However, since the bus electrodes 126 are arranged at a non-discharge gap, and address discharge is generated in the vicinity of the bus electrodes 126, wall charges generated in the address discharge are accumulated on and in the vicinity of the bus electrodes 126. Thus, the problem that address discharge is not smoothly transferred to sustaining discharge still remains unsolved.